1. Technical Field
The invention relates to a light-emitting device such as a light-emitting diode (LED), and particularly to an electrode structure of a semiconductor light-emitting device.
2. Description of the Related Art
There have conventionally been configurations of the electrode structure of an LED as noted below. Specifically, a semiconductor light-emitting device is described in Japanese Laid-open Patent Publication No. 2002-217450 (referred to as Patent Document 1 hereinafter). In this device, an electroconductive reflective film is formed between an electroconductive support substrate and a semiconductor layer that includes a light-emission layer, and which is provided with an island-shaped alloy contact layer periodically arranged on the electroconductive reflective film. It is noted in the description that such a configuration makes it possible to obtain a structure in which an ohmic contact region having low contact resistance and a highly optically reflective region having high optical reflectivity are repeated periodically on the interface between the semiconductor layer and electroconductive reflective film.
It is noted in the description of Japanese Laid-open Patent Publication No. 2008-282851 (referred to as Patent Document 2 hereinafter) that a plurality of ohmic contact electrodes are provided to a reflective film formed between a semiconductor layer and a Si support substrate, wherein each of the ohmic contact electrodes is arranged so that the distance from a surface electrode provided on the light extraction surface is the same. It is noted in the description that such a configuration makes it possible to inject the electric current uniformly into each of the ohmic contact electrodes and to prevent current concentration and an increase in forward voltage.
The light-emission efficiency of an LED depends on the current density injected into the light-emission or active layer. Specifically, when the current density increases, the carrier injected into the active layer overflows, causing the carriers contributing to light emission to decrease, and the light-emission efficiency to decrease. Furthermore, localized current concentration brings about electrical field concentration and heat generation, causes more crystal defects to grow, and adversely affects reliability.
In the case of the electrode structure described in the Patent Document 1, a uniform distance cannot be established between each of the island-shaped electrodes arranged in a distributed manner on the electroconductive reflective film and the surface electrode provided on the light extraction surface. Therefore, the current is concentrated in a path having the minimum distance between the electrodes, and light distribution becomes nonuniform.
In the case of the electrode structure described in the Patent Document 2, it seems that uniformity or inhomogeneity of the current density can be prevented because the distance between the surface electrode and each of the ohmic contact electrodes is the same. If, however, there is a slight difference in resistive components between the electrodes, the current will be concentrated in lower-resistance portions, creating a nonuniform light-emission distribution and adversely affecting reliability. A difference in resistive components between the electrodes may be attributed, for example, to a positional shift of an electrode caused in the production process, a difference in the sizes of ohmic contact electrodes arranged on a reflective surface in a distributed manner, a difference in contact resistance between an electrode and a semiconductor film, and other factors. Completely eliminating these factors is very difficult.
A method is also used in which the resistance of a semiconductor film is reduced by increasing the thickness of the semiconductor film, whereby current diffusion is facilitated and a uniform current density distribution is achieved. Using a thicker semiconductor film increases the amount of light absorption in the semiconductor film, resulting in reduced light-emission efficiency and increased production time and material cost. Production costs increase as a result.
Furthermore, a uniform current density distribution can be obtained by increasing the surface area of the electrodes, but increasing the surface area of the electrodes on the light extraction surface side will cause a decrease in the efficiency of light extraction.